A thermometer based on the Coulomb blockade and its principles has been described in the journals Physical Review Letters, Vol. 73, No. 21, Nov. 21, 1994, pp. 2903-2906; Applied Physics Letters, 67(14), Oct. 2, 1995, pp. 2096-2098; and Journal of Low Temperature Physics, Vol. 101, Nos. 1/2, October 1995, pp. 17-24. In the sensor of the thermometer there are several tunnel junctions arranged as a sequential chain. The phenomenon known as the Coulomb blockade causes a conductance drop at the zero point of the bias voltage, the characteristic quantities of which depend on the temperature. The method is based on the joint effect of the charging energy E.sub.c =e.sup.2 /2C, where C is the capacitance of the junction in question, of the very small capacitor formed by the small metallic island isolated by the tunnel junctions, and the thermal energy k.sub.B T. It becomes apparent that when the voltage-current characteristic curve of the chain of tunnel junctions is measured, more precisely as a function of the voltage affecting its dynamic resistance over the tunnel junction, a zero-center peak can be observed, the width of which is directly proportional to the temperature, and its absolute value corresponds to that calculated theoretically with a very high degree of accuracy. This property makes the thermometer a primary thermometer. On the other hand, the height of the peak is in inverse proportion to the temperature, which in turn provides a secondary thermometer.
Primary thermometers are rare, particularly at low temperatures. Some few in this category are the (ideal) gas thermometer at a temperature of more than 3 K, the nuclear orientation thermometer at temperatures of 3-50 mK, and the noise thermometer. A thermometer based on CB-tunneling (CBT), offers several advantages compared with those.
In practice, there are, however problems in CBT applications. First, the total impedance of the sensor becomes very great. In order that the theory can be realized in the tunnelling event itself as accurately as possible, the resistance R.sub.T of each tunnel junction must be clearly greater than the quantum resistance R.sub.K .tbd..h slashed./e.sup.2 .noteq.4 k.OMEGA.. In practice, R.sub.T .noteq.20 k.OMEGA. has been shown to be sufficient. On the other hand, in order that the a detrimental effect of the ends of the tunnel junction chain on the operation of the sensor is minimized, there must be at least N.noteq.20 junctions in the series. These two conditions combined give a "good" sensor minimum impedance (resistive) of Z.sub.min .noteq.400 k.OMEGA.. Such a high level of impedance causes problems, especially if the distance between the sensor and the electronics controlling it, and with it the capacitance of the measurement leads, increase.
Secondly, the temperature range of the sensors should be increased. The optimal temperature of the tunnel junction chain is determined by the capacitance C of the tunnel junctions. The temperature range of a sensor can nowadays be brought to slightly less than two decades. The upper limit temperature is determined by the signal-noise ratio of the measurement, in other words, the smallest measurable conductance peak. The lower limit temperature of each sensor is for its part determined by the fact that the sensor will no longer operate, as might be expected from the simple series development, if the relative height of the peak grows so that .DELTA.G/G.sub.T .gtoreq.0.2.
Thirdly, a frequent problem at temperatures of less than 1 K is the fact that the current flowing through the chain heats the sensor, so that the electron temperature, which is being measured here, rises above the temperature of the substrate.
A fourth type of a problem worth mentioning, is that particularly relating to the present material, aluminum. First, aluminum is a superconducting metal at temperatures of less than 1K. Because the sensor only works as a normal metal, a powerful magnetic field of about 0.5 T is required, by means of which the aluminum can be kept normal at ever lower temperatures. Secondly, at temperatures greater than 50 K the limiting factor becomes the height of the Al/AlO.sub.x /Al tunnel barrier, which is about 2 eV.
The invention is intended to resolve the above problems.